This invention relates to atomic emission spectroscopy and more particularly to an atomic emission spectrometer and technique for multi-element measurement of elements in a sample.
Atomic absorption spectroscopy (hereinafter "AAS") is a known procedure for measuring the concentration of a certain element in a sample. AAS utilizes the fact that atoms absorb light at certain wavelengths characteristic of the particular element. Atoms emit light in the form of a line spectrum when excited and the line spectrum is characteristic of the respective element. Correspondingly, the atoms absorb light only at the wavelengths of this line spectrum.
In AAS, an atomizing device such as a flame burner or a graphite furnace for electrothermal atomization is utilized to generate an atomic vapor of the sample in which the atoms of the sample are present in their atomic state. A measuring light beam is normally generated by a hollow cathode lamp and consists of light with the line spectrum of the looked-for element. This measuring light beam is passed through the atomic vapor and is subjected to absorption according to the amount of the looked-for element in the sample. The other components of the sample, at least theoretically, do not influence the measuring light beam because their absorption lines do not coincide with the line spectrum of the measuring light beam. The measuring light beam impinges on a photoelectric detector and the concentration of the looked-for element is determined from the detector signal after suitable processing and calibration. Conventionally, a photomultiplier is used as the detector in atomic absorption spectrometers.
A certain element concentration range is required for the absorption measurement. A concentration of the looked-for element which is too high causes virtually complete absorption of the measuring light beam such that measurement is no longer possible. In AAS, an optimal concentration range can be achieved by dilution if necessary because only one element at a time is being determined.
A disadvantage of AAS is that it is not suited for a multi-element measurement of a large number of elements. In AAS, the elements can only be determined one by one, i.e., one after the other. Therefore, another known analytical procedure is to measure the emission of a sample rather than its absorption, i.e., atomic emission spectroscopy, which facilitates multi-element measurement.
In atomic emission spectroscopy, plasma burners are often used as the atomization and excitation device in atomic emission spectroscopy. In plasma burners, an emerging inert gas is inductively transformed to a plasma of high temperature and the sample is led into this plasma. In another prior art atomization and excitation device, a sample is electrothermally dried and ashed in a graphite furnace similar to the graphite tubes used in AAS. The graphite furnace is then evacuated and an inert gas is introduced. Subsequently, an electrothermal atomization of the sample is effected. A gas discharge is caused in the mixture of inert gas and sample vapor by an anode such that the graphite tube operates as a hollow cathode lamp. The graphite tube serves as a hollow cathode.
A spectrum of the emitted light is generated by means of a polychromator. It is known to scan such a spectrum by means of a series detector or "detector array" consisting of a plurality of photodetectors. The entire spectrum is detected which results in a large amount of data and the signal processing is correspondingly complex.
A polychromator is known in which a dispersion is effected in high order in a first direction by an echelle grating. The different orders overlap and a dispersion is effected in a second direction perpendicular to the first direction by a dispersion prism whereby the different orders are separated. This results in a two-dimensional spectrum with very high resolution in a focal plane.
In the prior art polychromator, a mask with apertures at the location of the spectral lines of the spectrum which are characteristic of a certain element is arranged in the focal plane. These apertures are arranged to accommodate light pipes, each of which is guided to an associated photomultiplier. The number of available photomultipliers and thus the number of elements which can be analyzed simultaneously is therefore necessarily limited due to cost considerations.
In atomic emission spectroscopy for multi-element measurement, i.e., simultaneous measurement of several elements, the problem arises that the different spectral lines observed can have largely varying intensities. This may be due to the fact that the different elements which are associated with the individual spectral lines are contained in the sample in quite different concentrations. It may also be due to the fact that the different spectral lines can have quite different intensities. This problem is a quite different situation than that noted previously with respect to AAS where only one single spectral line of one single element is used at a time for measuring the concentration of this element. In the AAS situation with substantially constant intensity of the measuring light beam, it is possible to place the absorption of the atomic vapor into an optimum measuring range by well-defined dilution of the sample. In atomic emission spectroscopy, however, a dilution of the sample to place the spectral line of a first element into a favorable intensity range could push even the strong spectral lines of a second element below the detection limit.
Photomultipliers have the advantage of permitting a change of sensitivity by changing the applied voltage. Therefore, a large dynamic range as required for a multi-element measurement can be attained but preliminary adjustment is required. In an unknown sample, the concentrations of the different elements to be measured are not initially known and therefore, the intensities of the various observed lines are also not known. Consequently, checks are required before the actual measurement to permit adjustment of the voltages of the multipliers in accordance with such checks.
The dynamic ranges of photodiodes or similar semiconductor photodetectors are too small to permit adaptation to the different intensities of the spectral lines which occur in the multi-element measurement of atomic emission spectroscopy.
Accordingly, it is an object of the present invention to provide an atomic emission spectrometer and technique which overcomes many of these disadvantages and deficiencies.
Another object of the invention is to provide an atomic emission spectrometer for multi-element analysis which affords simultaneous measurement of a relatively large number of elements and wide dynamic range detection without pre-measurement adjustment.
Another object of the invention is to provide such an atomic emission spectrometer which is economical to construct.
A further object of the invention is to provide an atomic emission spectroscopy detection technique for multi-element measurements which accomplishes wide dynamic range line detection while facilitating the use of low cost detector elements.
Other objects will be in part obvious and in part pointed out more in detail hereinafter.
Accordingly it has been that the foregoing and related advantages are attained in an atomic emission spectrometer for multi-element measurement which includes an apparatus to atomize a sample and excite the atoms to emit characteristic spectral lines, a dispersion assembly to generate a spectrum of characteristic spectral lines, a photodetector assembly for simultaneously sensing the intensity of spectral lines of a plurality of elements and processing circuit means for determining the concentrations of the plurality of elements. The photodetector assembly has a plurality of semiconductor photodetectors positioned for simultaneously sensing a plurality of spectral lines for each element of the sample to be tested. An evaluation circuit selects the semiconductor photodetectors sensing the spectral line for each element which has an intensity optimally within the sensing range of the respective semiconductor photodetector.